There has been significantly growing interest in recent years in the transition metal-catalyzed metathesis of alkenes and alkynes (Trnka & Grubbs, 2001, Acc. Chem. Res. 34:18; Schrock & Czekelius, 2007, Adv. Synth. Catal. 349:55; Fürstner & Davies, 2005, Chem. Commun. 2307; Zhang & Moore, 2007, Adv. Synth. Catal. 349:93; Mori, 2007, Adv. Synth. Catal. 349:121; Astruc, 2005, New J. Chem. 29:42.). The synthetic potential of the latter, however, is much less explored even after it was used to prepare arylene ethynylene polymers (Bunz, 2001, Acc. Chem. Res. 34:998; Mang & Moore, 2004, Macromolecules 37:3973; Fischer & Nuckolls, 2010, Angew. Chem. 122: 7415; 2010, Angew. Chem. Int. Ed. 49:7257) and macrocycles (Zhang & Moore, 2006, Angew. Chem. 118:4524; 2006, Angew. Chem. Int. Ed. 45:4416; Zhang & Moore, 2005, J. Am. Chem. Soc. 127:11863; Zhang & Moore, 2004, J. Am. Chem. Soc. 126:12796; Ge et al., 2000, Angew. Chem. 112:3753; 2000, Angew. Chem. Int. Ed. 39:3607; Johnson II et al., 2007, Org. Lett. 9:3725; Jiang & Tew, 2008, Org. Lett. 10:4393), and in natural product synthesis (Fürstner & Davies, 2005, Chem. Commun. 2307; Micoine & Fürstner, 2010, J. Am. Chem. Soc. 132:14064).
Typically, the metal-alkylidyne catalysts for alkyne metathesis contain a metal-carbon triple bond and alkoxide/phenoxide/siloxide/amide ligands (Schrock, 2002, Chem. Rev. 102:145), and their catalytic activity can be tuned through judicious ligand design. Coordination of small molecules, in particular 2-butyne (a common metathesis byproduct), to the hexavalent Mo-alkylidyne complex, is an interfering reaction that leads to undesired alkyne polymerization (through the ring expansion mechanism, requiring two open substrate-binding sites) as well as nonproductive reaction pathways.
Polyhedral oligomeric silsesquioxane (POSS) and silica are the only reported ligands so far that can overcome this long-standing problem (Weissman et al., 2006, Angew. Chem. 118:599; 2006, Angew. Chem. Int. Ed. 45:585, Cho et al., 2006, J. Am. Chem. Soc. 128:14742; Gauvin et al., 2007, Dalton Trans. 3127). However, the siloxane-based approach lacks tunability in the catalyst structure, thus making it difficult to study the structure-activity relationship of the catalyst and tune its activity.
There is a need in the art for the identification of a novel ligand that can be used to generate highly active and selective catalysts for alkyne metathesis. The activity of these ligands should be tunable as to afford good metathesis activity and functional group tolerance. The present invention fulfills these needs.